Influence of surface anisotropy on turbulent flow over irregular roughness

Busse, A. and Jelly, T. O. (2020) Influence of surface anisotropy on turbulent flow over irregular roughness. Flow, Turbulence and Combustion, 104, pp. 331-354. (doi: 10.1007/s10494-019-00074-4)

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The influence of surface anisotropy upon the near-wall region of a rough-wall turbulent channel flow is investigated using direct numerical simulation (DNS). A set of nine irregular rough surfaces with fixed mean peak-to-valley height, near-Gaussian height distributions and specified streamwise and spanwise correlation lengths were synthesised using a surface generation algorithm. By defining the surface anisotropy ratio (SAR) as the ratio of the streamwise and spanwise correlation lengths of the surface, we demonstrate that surfaces with a strong spanwise anisotropy (SAR < 1) can induce an over 200% increase in the roughness function ΔU+, compared to their streamwise anisotropic (SAR > 1) equivalent. Furthermore, we find that the relationship between the roughness function ΔU+ and the SAR parameter approximately follows an exponentially decaying function. The statistical response of the near-wall flow is studied using a “double-averaging” methodology in order to distinguish form-induced “dispersive” stresses from their turbulent counterparts. Outer-layer similarity is recovered for the mean velocity defect profile as well as the Reynolds stresses. The dispersive stresses all attain their maxima within the roughness canopy. Only the streamwise dispersive stress reaches levels that are comparable to the equivalent Reynolds stress, with surfaces of high SAR attaining the highest levels of streamwise dispersive stress. The Reynolds stress anisotropy also shows distinct differences between cases with strong streamwise anisotropy that stay close to an axisymmetric, rod-like state for all wall-normal locations, compared to cases with spanwise anisotropy where an axisymmetric, disk-like state of the Reynolds stress anisotropy tensor is observed around the roughness mean plane. Overall, the results from this study underline that the drag penalty incurred by a rough surface is strongly influenced by the surface topography and highlight its impact upon the mean momentum deficit in the outer flow as well as the Reynolds and dispersive stresses within the roughness layer.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Busse, Professor Angela and Jelly, Mr Thomas
Authors: Busse, A., and Jelly, T. O.
College/School:College of Science and Engineering > School of Engineering
College of Science and Engineering > School of Engineering > Autonomous Systems and Connectivity
Journal Name:Flow, Turbulence and Combustion
Publisher:Springer Verlag
ISSN (Online):1573-1987
Published Online:20 November 2019
Copyright Holders:Copyright © 2019 The Authors
First Published:First published in Flow, Turbulence and Combustion 104:331–354
Publisher Policy:Reproduced under a Creative Commons License

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
712071Fluid dynamic properties of irregular, multi-scale rough surfacesAngela BusseEngineering and Physical Sciences Research Council (EPSRC)EP/P004687/1ENG - AEROSPACE SCIENCES
739531Surface-specific Moody diagram: A new paradigm to predict drag penalty of realistic rough surfaces with applications to maritime transportAngela BusseEngineering and Physical Sciences Research Council (EPSRC)EP/P009875/ENG - AEROSPACE SCIENCES